The A/D reference files may contain multiple look-up tables, which depend on the Bay 3 temperature (stored as degrees Celsius in the WBA3PCTM keyword of the observation). The first row in the A/D reference file consists of temperatures, while subsequent rows contain the lookup tables. That is, the second pixel in row one is the Bay 3 temperature associated with the second row A/D fixup values, the third pixel in row one is the Bay 3 temperature for the lookup table in the third row, and so on. However, since the Bay 3 temperature remained stable, only one lookup table is contained with the A/D reference file.
Note that the STSDAS task mka2d can be used to generate a lookup table. However, a table of the 48-bit errors are required as input (see OV/SV Report, Faber et al., 1992, Chapter 1).
45.3.6 Preflash/CTE File
A preflash was applied to circumvent the low level non-linearity in the CCDs: during readout, very low-level charge (< 250 e- /pixel) was inefficiently transferred, smearing faint images. In addition, some charge (~50e-) was completely lost, which adversely affected the photometry of even uncrowded fields. The preflash, which was requested by the observer, was done by imaging the shutter through a red filter, thereby imposing a low-level signal (~30 to 40 e-/pixel) on the image before the science exposure began.
st> engextr w0ts0n02t.x0h outtable=.The example above would write to the screen the values stored in rows 77-78 of the file w0ts0n02t.tab which contain the shutter status at the end of the image readout. Assuming that 1) there were no interruptions or multiple exposures before the readout, 2) SERIALS=NO, and 3) the observation is shorter than 300 seconds, the preflash shutter is the one opposite the shutter marked as closed in the table (e.g., if A is marked as closed in the extracted engineering table, B was used during the preflash).
st> tdump w0ts0n02t row=77-78
For exposures longer than 300 seconds, shutter B always closed the end of the exposure, regardless of the shutter used for preflash, so that the above algorithm fails-however, the difference between the A and B shutter blade preflashes was relatively small, less than about 0.5 DN. Shutter B was usually used to preflash long exposures, and that is the assumption used in calibrations. If the shutter blade is likely to be important for the observations, contact the STScI help desk (email@example.com ) for help in determining the preflash shutter.
Note that the DARKTIME used in pipeline processing is only an approximation.
For long exposures or data requiring very accurate dark correction, the darktime
should be recalculated (see WF/PC-1 Instrument Science Report 93-01), inserted
into the DARKTIME keyword, and then the images should be recalibrated. There
was a problem in older data with the DARKTIME being improperly set for interrupted exposures; in these cases, the darktime must be manually computed from
the WEXPODUR keyword and the DARKTIME keyword updated before recalibrating (see "Recalibrating WF/PC-1 Data" on page 45-20).
The data are multiplied by a normalized, inverted flatfield in order to remove any pixel-to-pixel variations in the cameras. Flatfields used in the pipeline calibration were generated from sets of earthcals (observations of the bright earth). The general procedure for creating a pipeline flatfield was to combine a chosen set of earthcals using the STSDAS task streakflat, then normalize and invert using normclip. The code allows for the fact that some of the earthcals had streaks due to features on the earth being smeared by HST motion in the images.
Care must be taken in choosing the best flatfield to use; flatfields generated from
earthcals taken with the neutral density filters (F122M or F8ND) can contain gradients of 20-30%. See "Choosing Among Available Flats" on page 45-15 and
"Flatfield Anomalies" on page 46-7 for details on the available flatfields.
45.3.9 Photometry Keywords
The photometry keywords are listed in Figure 45.3, below; the first two keywords are in the ASCII header (both d0h and c0h) while the last five keywords are group parameters (use the IRAF tasks imheader or hedit to examine the group keywords). (See "Converting Counts to Flux or Magnitude" on page 3-15).
Figure 45.3: Photometry -Keywords
DOPHOTOM= `YES ` / Fill photometry keywords: YES, NO, DONEThe PHOTTAB keyword is set by the PODPS pipeline during generic conversion; all other photometry keywords are blank or contain values of zero in the .d0h files. If the image is being recalibrated, use the StarView Calibration screens and consult the Reference File Memo on WWW for a listing of new photometry tables available for use.
PHOTTAB = `wtab$cbj15281w.cw0' / name of the photometry calibration table
`PHOTMODE' / Photometry mode (for example, PC,5,F,DN,F1042M,OPEN,CAL)
`PHOTFLAM' / Inverse Sensitivity (erg/sec/cm2/Å for 1 DN/sec)
`PHOTPLAM' / Pivot wavelength (angstroms)
`PHOTBW ` / RMS bandwidth of the filter (angstroms)
`PHOTZPT' / Photometric zeropoint (magnitude)
Setting DOPHOTOM to "YES" instructs calwfp to populate the photometry-related keywords; no operations are performed on the actual images. The first step is to form the PHOTMODE keyword based on the values of other keywords (CAMERA, DETECTOR, FILTNAM, etc.) in the header. The calwfp task then searches the photometry table specified in PHOTTAB for the matching mode; the PHOTFLAM, PHOTZPT, PHOTPLAM, and PHOTBW values in the table are written into the c0h group keywords (see Table 44.3). If no matching PHOTMODE is found in the table specified by PHOTTAB, the keywords remain blank or zero. For more information about improving the photometric calibration, see "Improving the Photometric Calibration" on page 46-17.
If this operation is requested (by setting DOHISTOS=YES), histograms of the raw data, the A/D corrected data, and the final calibrated output data are created and stored in the .c2h/.c2d image. This is a multigroup image with one group for each group in the calibrated data file. Each group contains a 3-line image where row 1 is a histogram of the raw data values, row 2 is a histogram of the A/D corrected data, and row 3 is a histogram of the final calibrated science data.
45.3.11 Data Quality Files
Each calibrated science dataset contains a data quality file (.c1h for WF/PC-1). The calwfp software will bit-wise OR all of the raw data quality files (.q0h, .q1h) with the reference file data quality files (.b2h, .b3h, etc.), in order to generate the calibrated science data quality file (.c1h). The flag values used are defined in Table 45.2. The final calibrated data quality file (.c1h) should be examined (for example, using SAOimage or imexamine) to identify which pixels may be bad in the science image.
The bad pixels flagged in the .c1h file are not fixed up in the .c0h file. The
-STSDAS task wfixup can be used to interpolate across bad pixels in the science
image; note however, that this results in a loss of information and can have
adverse effects on any photometry using those pixels.
WF/PC-1 Data Quality Flag Values
Reed-Solomon decoding error
Calibration file defect
Permanent camera defect
A/D converter saturation
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